Audio frequency transmission circuit



Feb. 16, 1937. o, FARNHAM 2,070,971

AUDIO FREQUENCY TRANSMISSION CIRCUIT Filed May 18, 1935 mnsmllssfonFrequeflcy 3mm;

Patented Feb. 16, 1937 UNITED) STATES PATENT orrlcs a AUDIO rarounitrtitamsmssrorz I n I Paul 0. Farnham, Boonton, N. J., assignor, by mesneassignments, to Radio Corporation of America, New York, N. Y., acorporation of Delaware Application May 1a, 1933, Serial No. 671,773

13 Claims. (Cl. 179-1) This invention relates to audio frequencytransmission circuits and more particularly to circuits includingadjustable elements for controlling the transmission efficiency.

5 The recent trend in radio receiver design has been towards a higherquality of audio reproduction and this extension of the audio frequencyband has resulted, when some of the known volume control systems areemployed, in a variation of tone quality with changes in the audiolevel.

An audio transmission system which has a substantially equaltransmission efficiency over a wide frequency band, for all adjustmentsof the outputlevel, has the apparent effect of suppressl6 ing the higherand lower frequencies at low output levels since the ear is not equallysensitive over the entire band when the output or room level ofreproduction is reduced.

An, object of the invention is to provide an audio frequencytransmission system in which changes in output level are automaticallyaccompanied by an adjustment of the frequency-transmission efiiciency ofthe system. An object is to provide an audio frequency amplifier circuitincluding circuit elements adjustable to vary the output level by afrequency-selective control of the transmission efficiency. Moreparticularly, an object of the invention is'to provide an audiofrequency transmission system of the type in 30 which the primary of atransformer is coupled through a condenser to an adjustable tap on aninput resistance, and in which the transformer circuit has resonancepeaks at the ends of the audio frequency band which is to be reproduced.These and other objects and advantages of the invention will be apparentfrom the following specification when taken with the accompanyingdrawing, in which:

Fig. 1 is a fragmentary circuit diagram of an 0 audio frequencytransmission system embodying the invention; and

Fig. 2 is a curve sheet showing the relation between frequency andtransmission efficiency at different adjustments of the output level ofthe 45 system.

In the drawing, the reference numeral l identifies a vacuum tube whichmay be an audio amplifier of a radio receiver or other transmissionsystem, or'the detector of a-radio receiver. The

50 plate circuit of tube l includes a voltage divider or resistance Rhaving an adjustable tap for impressing any desired portion of theavailable output voltage across theprimary winding L1 of the transformerwhich couples the tube I to the folup in the transformer.

lowing load circuit. A blocking condenser C1 is included in' the leadfrom winding L1 to the tap on'resistance R to permit the grounding ofthe other terminal of the winding. An amplifier tube 2 is illustrated asthe audio load connected across the secondary winding L2, ,but it willbe apparent that the .transformer may work into other types of loadcircuits. In any event, a capacity C2 is effectively in shunt with thesecondary and, as indicated by the dotted line illustration, thecapacity may be provided solely by the distributed capacity of thesecondary and the input capacity of the load circuit.

As the tap is adjusted towards the low potential terminal of resistanceR, the reduction in the magnitude of the section R1 of the resistancebrings the capacities C1, 02 into play to alterthe transmissioncharacteristics of thecircuit. appropriate design of the'transformer,this effect is not detrimental but results in a desirable regulation offidelity withchanges in output level. Since the car has approximatelythe same sensitivity for all audio frequencies at high output levels butis less sensitive to the lower and higher frequencies at low levels, aneffective tone compensation for changing output levels requires auniform transmission for all frequencies at high levels and a greaterreduction of the medium than of end frequencies when the output level islowered.

The invention provides means for automatically effecting this tonecompensation by the use of a transformer having suchprimary and leakagereactances that the transformer resonates with capacity C1 at the lowerend of the transmitted frequency band and with capacity C1 in serieswith capacity C2 at the higher end of the band. At high output levels,the resonance effects are negligible since resistance R1 is included inthe resonant circuits and, for high output levels, is of substantialmagnitude. As the value of resistance R1 is reduced to lower the outputlevel,

the resonance effects become increasingly important and thus prevent thetransmission efficiency at the ends of the audioband from falling asrapidly as thetransmission efliciency for fre-- quencies at the centralsection of theband.

The invention presents the further advantage that a transformer ofappropriate design requires a primary winding of relatively lowimpedance, and therefore permits a substantial voltage step- The turnsratio of windings L1, L2 may be of the order of 1 to 6.

. The following data is given asan example of appropriate values whichmay be used to obtain resonance effects at about 100 to 4000 cycles.

The tone compensation which is obtainable with such a system isrepresented graphically in Fig. 2, in which curve A shows the relationbetween transmission and frequency when the tap is moved to the highpotential terminal of resistance R. In obtaining the values for plottingcurve A, voltages of different frequencies but of a constant magnitudewere impressed across resistance R through a series resistance of 10,000ohms equal to the output or plate resistance of tube I, and theresulting output voltages across winding L2 were measured, the outputvoltage for 1000. cycles being arbitrarily taken as unity of the scaleof transmission values. For'this maximum output level, there was anapproximately equal transmission of all audio frequencies between 100and 4000 cycles.

Curves B, C and D are similar curves representing thetransmission-frequency characteristic of the network when the tap wasadjusted to make -R1 equal to 0.4, 0.2 and 0.1, respectively, ofR.

It will be noted that a frequency selective adjustment of thetransmission efliciency was obtained, and that the characteristic curvebecame more sharply peaked at the ends as the output level was lowered.I

-The degree to which the end frequencies are emphasized as the generaloutput level is lowered may be varied by appropriate changes in therelative values of the transformer inductances and the capacities or bythe size of resistance R, relative to the plate resistance of tubeAlthough the series of curves show that the fidelity of transmissionvaries with the output level, the variation is in such sense as tocompensate for the frequency selective response of the human ear, andthe net result is that approximately the same fidelity of reproductionismaintalned for all output levels. r

It will be apparent that the invention is not limited to the particularvalues of circuit ele- "ments or to the particular frequency rangerecited in the above example.

I claim:

1. An audio frequency transmission system including an input resistanceacross which an audio frequency voltage may be impressed, an adjustabletap on said resistance, an audio frequency load circuit connectedbetween said tap and one terminal of said resistance, and means seriallyconnected between said tap and said load circuit and including theportion of the resistance between the adjustable tap and said terminalfor reducing the efficiency of transmission of low frequencies at a lessrapid rate than that of the reduction in the efficiency of transmissionof higher'frequencies when said tap isadjusted to lower the transmissioneifioiency.

2. An audio frequency transmission system including an input resistanceacross which an audio frequency voltage may be impressed, an adjustabletap on said resistance, an audio frequency load circuit connectedbetween said tap and one load circuit connected between said tap and oneterminal of said resistance, and means serially connected between saidtap and said load circuitv and including the portion of said resistancebe- .tween the tap and the terminal for reducing the efii'ciency oftransmission of frequencies at the respective ends of the transmittedaudio frequency band at a less rapid rate than that of the reduction inefi'iciency of transmission of frequencies at the central part of theband when the tap is adjusted to lower the transmission efficiency.-

d. An audio frequency transmission system comprising an input resistancehaving a tap adjustable along the same, a coupling transformer, meansconnecting the low potential terminal of the transformer winding to thelow potential terminal of said resistance to maintain the same at thesame audio frequency potential, a capacity connected between the otherterminal of said primary and the adjustable tap, and an audio frequencyloadcircuit connected across the secondary of said transformer, thetransformer resonating with said capacity at a frequency at the lowerend of the band of frequencies to be transmitted, the portion of saidresistance connected across the transformer winding acting to controlthe resistance effects of the transformer and capacity said resistanceeffects becoming increasingly greater as the value of said portion ofthe resistance is decreased.

5. An audio frequency transmission system as claimed in claim 4, incombination with capacitive means resonating with said transformer at afrequency at the upper end of the band of frequencies to be transmitted.q

6. A transmission system adapted to pass a band of audio frequencies,said system including an input resistance across which audio frequencyvoltages may be impressed, an adjustable tap on said resistance, acoupling network comprising reactive elements and resonant at twofrequencies between which said band is included,

and a load circuit, said coupling network having output terminals acrosswhich said load circuit is connected and having input terminals con--nected, respectively, to said tap and to one end of said inputresistance, and said reactive elements being effectively in series withthe portion of the said resistance included between said tap and thatend of said input resistance to which an input terminal is connected,whereby the adjustment of said tap to transfer less audio voltage tosaid coupling network'reduces the resistance in series with saidreactive elements to effect a lesser change in the transmission offrequency in the middle of said band than in the transmission offrequencies at the ends thereof.

7. In an audio frequency transmission system, the combination with avoltage divider comprising a resistance across which an audio frequencyvoltage maybe impressed, and a tap adjustable along said resistance tocooperate with one-terminal thereof as the output terminals of saidvoltage divider, of an audio frequency load circuit, and a reactivenetwork serially connected between said output terminals and said loadcircuit means including the portion of the resistance between the tapand said output terminal for acting upon the reactive network forreducing the efficiency of transmission of low frequencies at a lessrapid rate than that of the reduction in the efliciency of transmissionof higher frequencies when said tap is adjusted to transmit a lesserfraction of the total available audio frequency voltage to said loadcircuit.

8. In an audio frequency transmission systern, the combination with'avoltage divider com'-,

prising a resistance across which an audio frequency voltage may beimpressed, and a tap adjustable along said resistance to cooperate withone terminal thereof as the output terminals of said voltage divider, ofan audio fre uency load circuit, and a reactive network seriallyconnected between said output terminals and said load circuit meansincluding the portion of the resistance between the tap and said outputterminal for acting upon the reactive network for reducing theefficiency of transmission of high frequencies at a less rapid rate thanthat of the reduction in the efliciency of transmission of lowerfrequencies when said tap is-adjusted to transmit a lesser fraction ofthe total available audio frequency voltage to said load circuit.

9. In an audio frequency transmission system,

the combination with a voltage divider compris-' ing a resistance acrosswhich an audio frequency voltage may be impressed, and a tap adjustablealong said resistance to cooperate with one terminal thereof as theoutput terminals of said voltage divider, of an audio frequency loadcircuit, and a reactive network serially connected between said outputterminals and said load circuit, means including the resistance elementfor affecting the reactive network for reducing the efiiciency oftransmission of frequencies at the respective ends of the transmittedaudio frequency band'at a less rapid rate than that of the reduction inefliciency of transmission of frequencies at the central part of theband when said tap is adjusted to transmit a lesser fraction I of thetotal available audio frequency voltage to said load circuit. i

10. An audio frequency transmission system including an input impedancedevice across which an audio frequency voltage may be impressed,

an adjustable tap on said impedance device, an

audio frequencyload circuit connected between i the tap and one terminalof the impedance device and means comprising a circuit having resonanceeffects at low audio frequencies connected between the tap and the loadcircuit for reducing the efficiency of transmission of the low audiofrequencies at a less rapid rate than that of the reduction in theefliciency of transmission of.

higher frequencies when the, tap is adjustedto lower the transmissionefliciency, the degree of the resonance effects being controlled inaccordance with the resistance value of the impedance between the tapand the terminal of the impede ance.

11. An audio frequency transmission system including an input resistanceacross which an audio frequency voltage may be impressed, an

adjustable tap on the resistance, an audio fre-.

of .the reduction in the efliciency of transmission of higherfrequencies when the tap is adjusted to reduce the amount of energytransferred to the load circuit the amount of resistance between the tappoint and said terminal acting to determine the differences in the ratesof reduction.

12. An audio frequency transmission system including an input resistanceacross which an audio frequency voltage may be impressed, an adjustabletap on the resistance, an audio frequency load circuit'connected betweenthe tap and one terminal of the resistance whereby substantially anypart of the voltage drop across said resistance may be impressed uponthe load circuit in accordance with the-position of the tap on theresistance and means connected between the tap and the load circuit andincluding said resistance for reducing the efliciency of transmission offrequencies at the respective ends of the transmitted audio frequencyband at a less rapid rate than that of the reduction in the efliciencyof transmission of frequencies at the central part of the band when thetap is adjusted to reduce the amount of energy transferredto the loadcircuit the amount of resistance between the tap point and said terminalacting to determine the differences in the rates of reduction.

13. An audio frequency transmission system infrequency voltage may beimpressed, an adjustable tapon the resistance, an audio frequency loadcircuit connected between the tap and one terminal of the resistancewhereby substantially any portion of the voltage drop across thereeluding an input resistance across which an audio sistance may beimpressed upon the load circuit in accordancewith the portion of the tapon the resistance, a coupling network between the resistance end andload circuit comprising reactive elements, said coupling network beingresonant at two frequencies between which the band of audio frequenciesis included, said reactive elements being effectively in series with theportion of the said resistance included between the tap and saidterminal of the resistance,

the resistance value of said last named portion of the resistance actingto determine the effect of the coupling network upon the band offrequencies transmitted in such a way that the adjustment of said tap totransfer less audio voltage to the coupling network effects a differentchange in the transmission of frequencies in the middle of the band offrequencies and in the transmission of frequencies at the ends of saidband of frequencies.

PAUL O. FARNHAM.

